The present invention relates generally to the area of fluid handling and solid/liquid separation and, more particularly, to a horizontal scroll and screen centrifuge.
It is well known to use a scroll and screen centrifuge to separate a crystalline, a granular or a fibrous product from a liquid in which the crystalline, granular or fibrous product is carried. Scroll and screen centrifuges are typically used with a continuous process in which a combined liquid and solid material is continuously fed into and continuously discharged from the centrifuge. The continuous process is distinguished from a batch process in which a batch of the liquid and solid material is fed into the centrifuge; and the liquid and solid materials are separated. The centrifuge is stopped, and the separated materials are removed before another batch is loaded into the centrifuge. Both the continuous and batch separation processes are also considered to dry the solid material. The separation process is used in a wide range of applications including the production of soap powders, coal, gypsum, plastic granules, potash, salt, etc.
In a typical horizontal scroll and screen centrifuge, a scroll and basket assembly is frustoconical, that is, shaped like a frustum, and is mounted for rotation within a housing with respect to a generally horizontal axis of rotation. A frustoconical scroll has a tubular side wall with outwardly projecting helical screw flights and rotates within an independently rotating frustoconical screen and basket assembly. The scroll rotates at a slightly different angular velocity from the screen and basket assembly, so that the helical screw flights on the scroll pass close to an inner directed surface of the screen. The helical length of the screw flights varies in a range of from about a fraction of a revolution to several revolutions around the scroll member. A liquid containing solids to be separated therefrom is fed into a closed rearward portion of the scroll and the rotations of the scroll and the screen and basket assembly produce a centrifugal force causing the liquid to pass through openings in the screen thereby separating the liquid from the solids. The rotational velocity difference between the scroll and the screen and basket assembly causes the screw flights to push the solid material across the screen from the smaller, closed, rear end of the scroll flights to the larger open, forward end of the scroll from which the solid material is discharged. The processing time or retention time of the material in the centrifuge is controlled by the pitch of the helical screw flights on the scroll and the difference in angular velocities between the scroll and the screen and basket assembly. Retention time can also be influenced by the design of the feed openings in the scroll. The solid particles pass from the centrifuge to another processing station, a conveyor or a collection unit.
Using a centrifuge to separate a liquid from a solid material in oil mud presents numerous challenges. First, the centrifuge must often be located on an oil derrick platform at sea. As will be appreciated, it is very important that the centrifuge be as small as possible to minimize its footprint and consume the least possible area on the platform.
Second, the solid material separated from the oil mud must be removed from the oil rig platform for proper disposal, either by hauling it inland or, if properly processed and reduced to a low enough moisture level, disposed of at sea. Therefore, it is desirable that the centrifuge be very efficient in order to minimize the quantity of liquid in the solid material.
Third, a centrifuge mounted on an oil rig platform is exposed to the elements, and therefore, the design of a centrifuge intended for use on an oil rig platform must be rugged to withstand extended exposure to the elements.
Fourth, unlike many other environments in which a centrifuge is located, an oil rig platform does not have a crane readily available to lift various components, for example, the scroll, when the centrifuge is being serviced or the screen is being changed. Therefore, it must be possible to perform all of the routine maintenance and service on the centrifuge without the need of an independent and separate crane.
Fifth, a centrifuge separating oil mud on an oil rig platform is subjected to long periods of continuing use and therefore, must operate very reliably between scheduled down times. Further, such a centrifuge must be price competitive with other liquid/solid separation equipment.
Thus, there is a need for a centrifuge that provides an improved performance and is particularly suitable for separating liquid and solids in oil mud on an oil rig platform.
The present invention provides a horizontal centrifuge having a relatively small footprint and very efficient in operation. Further, the horizontal centrifuge of the present invention includes a self-contained scroll removal tool and is especially useful for separating oil mud on an oil rig platform.
According to the principles of the present invention and in accordance with the described embodiments, the invention provides a horizontal centrifuge for separating oil mud into a liquid and a solid material. The centrifuge has a basket rotatably mounted within a housing. The basket has a plurality of rings disposed between open and closed ends of the basket and a plurality of guide rails are circumferentially spaced with respect to the rings. The guide rails have respective lengths extending between the open and the closed ends of the basket, and the guide rails provide a plurality of respective bearing surfaces. A tubular screen is supported in the basket by the plurality of bearing surfaces of the guide rails. A scroll is mounted within the basket to rotate about the axis of rotation relative to the basket. The scroll has a tubular side wall and a plurality of helical screw flights extending outwardly from the tubular side wall between outer and inner ends of the scroll. Each of the plurality of screw flights has an edge moving in close proximity and relative to an inner surface of the screen. The scroll further has a feed opening proximate the rear end of the tubular side wall, and an infeed tube with an outlet extending into the outer end of the scroll. The infeed tube directs oil mud into the rotating scroll and through the feed opening. The helical screw flights of the scroll carry solid material to a solid material exit while liquid passes through the rotating screen and basket to a liquid exit.
In one aspect of this invention the tubular screen is made from six arcuate screen segments joined together. Using six arcuate segments provides the screen with greater circularity and allows the screen to better conform to the bearing surfaces on the guide rails, thereby making the screen more concentric with the basket and the scroll. A retaining ring or clamp ring urges the screen against the bearing surfaces, which helps maintain the roundness of the screen. The greater concentricity permits the edges of the helical screw flights to be positioned more closely to the inner surface of the screen. The net result is that the centrifuge is more effective at separating the liquid from the solid material, thereby resulting in lesser volume of solid material that must be transported for proper disposal. In another aspect of this invention, the bearing surfaces of the guide rods and the peripheral edges of the helical screw flights are machined surfaces to provide even greater concentricity between the scroll and the screen, thereby further improving the performance of the centrifuge.
In another embodiment of the invention, a horizontal scroll screen centrifuge of the type generally described above has a fabricated scroll. The fabricated scroll has a tubular side wall with outer and inner ends adjacent respective open and closed ends of the basket. The fabricated scroll further has a plurality of helical screw flights attached to and extending outwardly from the tubular side wall. Each of the plurality of screw flights has an edge moving in close proximity and relative to an inner surface of the screen. An end plate is attached at the inner end of the tubular side wall to form a generally closed scroll end, and an infeed plate is attached circumferentially inside the tubular side wall intermediate the inner and outer ends. A fabricated scroll is used instead of the known cast scrolls; and further, machining the edges of the screw flights as described above permits the fabricated scroll to achieve a superior performance than centrifuges using known cast scrolls. In addition, the fabricated scroll weighs less and is easier to handle during a scroll changing process.
In a further embodiment of the invention, a horizontal scroll screen centrifuge of the type generally described above further has a scroll changing tool mounted to the housing to support the scroll during a screen changing process. The scroll changing tool has a base mounted to an upper side of a centrifuge housing, and an overarm support mounted on the base and extending forward of the housing. In one aspect of the invention, the overarm support is pivotally mounted in the base. Thus, the scroll can be removed from the centrifuge, the screen replaced and the scroll reinstalled in the centrifuge without the need for a crane or other device separate from the centrifuge.